Method for synthesizing an essentially V2 O5 -free vanadium oxide

ABSTRACT

A new method for synthesising an essentially V 2  O 5  -free vanadium oxide having a mean vanadium oxidation state of at least +4 but lower than +5 from NH 4  VO 3  is disclosed. By this method NH 4  VO 3  is heated to a reaction temperature sufficient for thermal decomposition of NH 4  VO 3 , and at said reaction temperature, the pressure is kept on at least 0.5 MPa. The method enables production of single-phase V 6  O 13 , single-phase VO 2  as well as any mixture thereof in a remarkably simple manner, as it primarily involves careful control of temperature and pressure conditions. The synthesis method of the invention can thus be easily scaled up to any industrial requirement.

The present invention is concerned with a method for synthesising anessentially V₂ O₅ -free vanadium oxide having a mean vanadium oxidationstate of at least +4 but lower than +5 from NH₄ VO₃, said vanadium oxidepreferably consisting essentially of V₆ O₁₃, VO₂ or any mixture thereof.

Vanadium oxides having a mean vanadium oxidation state between +5 and +4have attracted much attention as potential active cathode materials insecondary lithium batteries. These materials are generally prepared fromNH₄ VO₃ (ammonium metavanadate).

Single phase V₂ O₅ is readily synthesised by heating NH₄ VO₃ in air (seee.g. K. C. Khulbe and R. S. Mann, Can. Jour. Chem, Vol. 53 (1975), p.2917). The lower oxides, especially V₆ O₁₃ (mean oxidation state +41/3)and VO₂ (mean oxidation state +4), are more difficult to obtain assingle-phase materials (see e.g. U. von Sacken and J. R. Dahn, J. PowerSources, Vol. 26 (1989), p. 461).

In J. Thermal Anal., Vol. 16 (1979) 659 Trau discusses the problem ofobtaining phase-pure V₆ O₁₃ under thermal decomposition of NH₄ VO₃without using added reduction reactants. The use of SO₂ as a reducingagent for V₂ O₅ to form V₆ O₁₃ is technically very complicated; it isdifficult to stop the reduction and obtain a homogeneous single-phase V₆O₁₃ powder. Trau instead suggests thermal decomposition in a stream ofpure nitrogen gas with slow heating to 500-550° C., and a post-treatmentwith boiling NaOH.sub.(aq) to remove V₂ O₅ as proposed by Yankelvich etal. in Ukr. Khim. Zh., Vol. 42 (1976) 659.

Brown et al. reports in J. Thermal Anal, Vol. 6 (1974) 529 the formationof lower oxides using an NH₃ atmosphere when decomposing NH₄ VO₃.

U.S. Pat. No. 4,486,400 describes a process for preparing stoichiometricV₆ O₁₃ by thermal decomposing ammonium metavanadate in a nitrogenatmosphere to produce nonstoichiometric V₆ O₁₃, followed by heating theobtained nonstoichiometric V₆ O₁₃ in an CO/CO₂ -atmosphere having acomposition giving an oxygen partial pressure equal to the oxygenpartial pressure over stoichiometric V₆ O₁₃ to form stoichiometric V₆O₁₃.

U.S. Pat. No. 4,035,476 describes the preparation of agglomeratedvanadium oxides of the formula V₂ O_(x), wherein x is between 3.8 and4.6, by thermal decomposition of ammonium polyvanadate ((NH₄)₂.O.3V₂O₅.nH₂ O) at a temperature of 600 to 900° C. and permitting the soliddecomposition products and reducing agents to react.

JP 62-197317 describes the production of V₆ O₁₃ or V₂ O₄ by introducingNH₄ VO₃, optionally mixed with ≦15% V₂ O₅, into a reaction vessel,heating to a temperature of 380 to 750° C. at a rate of 0.5 to 30 K/min,keeping this temperature for 30 min to 3 hours and keeping the pressureat about 3 atm or below by means of a pressure-control valve.

Impurities in the cathode material generally have a negative influenceon the overall battery performance, especially the battery capacitydeclines faster in the presence of impurities. In particular it is hasbeen shown that even small amounts of other VO_(x) phases, especially V₂O₅, has a negative influence on the lithium intercalating properties ofV₆ O₁₃ and VO₂.

Thus, although several methods for the manufacturing of lower vanadiumoxides are known there still exists a need for an improved synthesismethod for the preparation of lower vanadium oxides of high purity, i.e.essentially free of V₂ O₅.

Accordingly, it is an object of the invention to provide a method forsynthesising an essentially V₂ O₅ -free vanadium oxide having a meanvanadium oxidation state of at least +4 but lower than +5 from NH₄ VO₃,said vanadium oxide preferably consisting essentially of V₆ O₁₃, VO₂ orany mixture thereof.

It has been shown that this object is accomplished by a method describedabove, in which NH₄ VO₃ is heated to a reaction temperature sufficientfor thermal decomposition of NH₄ VO₃, and in which, at said reactiontemperature, the pressure is kept on at least 0.5 MPa.

It is a further advantage of the method according to the invention thatit does not involve any time-consuming and thus costly separationprocesses. Nor does it involve the use of added reducing agents in orderto obtain the desired purity, but only carefully controlled temperatureand pressure conditions. The synthesis method of the invention can thusbe easily scaled up to any industrial requirement.

The formation of vanadium oxides from NH₄ VO₃ involves two processes:the decomposition of ammonium metavanadate, followed by the reduction ofthe formed vanadium species. These processes can proceed simultaneouslyand the reaction routes are often quite complicated.

The formation of V₆ O₁₃ from NH₄ VO₃ presumably proceeds through areduction of vanadium from oxidation state +5 resulting in formation ofN₂ and the lower oxide (V₆ O₁₃) having a mean oxidation state of +41/3,according to the following total reaction scheme:

    18NH.sub.4 VO.sub.3 →3V.sub.6 O.sub.13 +14NH.sub.3 +15H.sub.2 O+2N.sub.2                                                (1)

It is very likely that the same type of reaction route is followed whenVO₂ is formed, thus giving rise to the following total reaction scheme:

    18NH.sub.4 VO.sub.3 +18VO.sub.2 +12NH.sub.3 +18H.sub.2 O+3N.sub.2(2)

It has surprisingly been shown that the structure of the vanadium oxideproduced by carrying out the process according to the invention can becontrolled in a remarkably simple manner by varying the pressure underwhich the product is synthesised:

When the pressure is kept within the range of 0.5 to 2.5 MPa, preferably1.0 to 2.0 MPa, e.g. about 1.5 MPa, the product obtained is single phaseV₆ O₁₃. At pressures below 0.5 MPa, V₂ O₅ is produced together with V₆O₁₃. At pressures above 2.5 MPa VO₂ is formed together with V₆ O₁₃. Whenthe desired product is single phase VO₂ the pressure should be at least3.5 MPa, preferably in the range of 3.5 MPa to 7.0 MPa.

In the present context the expression "single phase" is used todesignate vanadium oxides which according to X-ray analysis containvirtually no other crystal phases (less than the detectability limit forXRD of approximately 2%) than the predominant one.

The reaction temperature should be selected so that efficientdecomposition of NH₄ VO₃ occurs. Accordingly, NH₄ VO₃ is preferablyheated to a temperature of at least 250° C., more preferably to atemperature in the range of 300 to 800° C., even more preferably 425 to550° C.

The applied heating rate is advantageously lower than 2 K/min.Preferably the heating rate is in the range of 0.1 to 2 K/min, morepreferably in the range of 0.5 to 1 K/min.

The period at which the NH₄ VO₃ is kept at the reaction temperature mayvary according to the desired end product. For synthesis of single phaseV₆ O₁₃ a reaction period of 10 s to 24 h is preferably employed, whereasfor synthesis of single phase VO₂ a reaction period of 2 h to 5 d ispreferably employed.

The heating is preferably performed under such conditions that the soliddecomposition product and the produced decomposition gas, including NH₃,from the NH₄ VO₃ starting material are permitted to react.

In a preferred embodiment such conditions are ensured by heating the NH₄VO₃ in a closed reaction chamber, preferably equipped with means forcontrolling the pressure in the chamber, such as an adjustable reliefvalve.

In this embodiment the reaction chamber is preferably filled with anamount of NH₄ VO₃ powder corresponding to 1/2 to 9/10 of the reactionchamber volume, so that the produced decomposition gas efficientlydisplaces the air in the reaction chamber.

The invention will be further described with reference to examples andthe drawing in which:

FIG. 1 shows an apparatus for performing the process according to theinvention;

FIG. 2 shows a real (upper curve) and a simulated (lower curve) X-raydiffractogram of a single-phase V₆ O₁₃ sample produced according to anembodiment of the invention; and

FIG. 3 shows a real (upper curve) and a simulated (lower curve) X-raydiffractogram of a single-phase VO₂ sample produced according to anotherembodiment of the invention.

The reaction chamber used for the synthesis of the vanadium oxideaccording to the invention is shown in FIG. 1. The chamber, tubes andall welds are made of stainless acid resistant steel. The chamber couldonly be opened and closed at the stainless steel high-vacuum coupling(CF 16, VACUTECH), which was sealed with replaceable copper gaskets, sixsteel screws (Unbraco M4) and 8 mm silver steel nuts.

The experimental set-up shown in FIG. 1 was used for the synthesis of V₆O₁₃ (Example 1). The desired overpressure was regulated with a reliefvalve (NUPRO R3A) fitted with viton standard seals and pre-set withexchangeable springs (NUPRO, blue K1-A). Prior to an experimental runall parts were cleaned at 60° C. in a basic detergent (Labkemi RBS 25)for 60 minutes in an ultra-sound bath and thereafter rinsed in water andethanol.

A slightly different experimental set-up was used for the VO₂synthesis(Example 2). The pressure was controlled with a blanking flange(Balzer DN 16 CF) mounted directly on the reaction chamber.

The final products produced according to the following examples 1 and 2were characterised in terms their phase-purity and degree ofcrystallinity by X-ray diffraction (XRD) using CuKα₁ radiation and aSTOE & CIE GmbH STADI powder diffractometer equipped with a curvedposition sensitive detector.

The mean oxidation state for the vanadium of the final products wasdetermined by titration. The V₆ O₁₃ and VO₂ products were dissolved in0.5M H₂ SO₄ and 3M HCl, respectively. The dissolved samples were firsttitrated up to V(+5) with a standard Ce(SO₄)₂.4H₂ O (Merck, 0.10M)solution, followed by titration down to V(+4) with a solution ofFeSO₄.7H₂ O (Merck, p.a.), controlled against standard KMnO₄ (Merck,0.02M). Three subsequent titrations were performed.

Grain-size distribution and morphology of the final products wereanalysed by scanning electron microscopy (SEM) on samples pressed onto acarbon film using a Zeiss DSM 960A scanning electron microscope.

EXAMPLE 1

Synthesis of single-phase V₆ O₁₃.

Using the apparatus shown in FIG. 1, single-phase V₆ O₁₃ was synthesisedin the following manner:

NH₄ VO₃ powder (Gesellschaft fur Electrometallurgie, MBH, 99.9%) wasintroduced in the chamber (having a volume of 940 cm³) in an amountcorresponding to 2/3 of the chamber volume. Then the reaction chamberwas sealed, connected to the relief valve arrangement, placed in atemperature controlled furnace and heated to a temperature of 500° C. ata heating rate of 0.5 K/min. The relief valve was adjusted so as tocontrol the pressure in the reaction chamber to 1.5 MPa. The temperaturewas kept at 500° C. for 1 minut. The chamber was then allowed to cool toroom temperature in an airstream and dismantled. The obtained dark(bluish-black) powder had sintered into agglomerates and had to bescraped out of the chamber.

The phase purity of the resulting product was checked by X-raydiffraction (XRD). In FIG. 2 is shown the X-ray diffractogram for theobtained V₆ O₁₃ product in the upper curve, while the lower curve is acorresponding simulated X-ray diffractogram. XRD showed that the V₆ O₁₃obtained was phase-pure and highly crystalline.

A mean vanadium oxidation state of 4.30 (±0.01) was determined bytitration.

SEM studies of the morphology showed needle-like crystals propagating inthe direction of the short b-axis in the monoclinic V₆ O₁₃ unit cell.However, very different lengths were observed; from 5 to 50 μm foradjacent crystals. The crystals compacted together on sintering to formspherical powder grains with a maximum dimension of up to 300 μm.

EXAMPLE 2

Synthesis of single-phase VO₂.

The reaction chamber shown in FIG. 1 was slightly modified for the VO₂synthesis, the flange being closed by a plate.

NH₄ VO₃ powder (Gesellschaft fur Electrometallurgie, 99.9%) wasintroduced in the chamber in an amount corresponding to 9/10 of thechamber volume. Then the chamber was heated in the same manner and atthe same rate as in Example 1 to a temperature of 500° C. and annealedat that temperature for 3 days. The obtained VO₂ powder (greyish-black)consisted of very fine grains and could easily be removed from thereaction chamber.

The phase purity of the resulting product was checked by XRD. In FIG. 3is shown the X-ray diffractogram for the obtained VO₂ product in theupper curve, while the lower curve is a corresponding simulated X-raydiffractogram. XRD showed that the VO₂ formed was phase-pure andcrystalline.

A mean vanadium oxidation state of 4.00 (±0.01) was determined bytitration.

SEM revealed bulky crystals (5 to 20 μm) in a quite homogeneous andnon-sintered powder.

In the following Table I the results from the analysis of the productssynthesised in Examples I and II are presented.

                  TABLE I                                                         ______________________________________                                                    Example                                                                       I          II                                                     ______________________________________                                        Compound      V.sub.6 O.sub.13                                                                           VO.sub.2                                           Crystal Structure                                                                            C 2/m           P 2.sub.1 /m                                                   a = 11.911(3)Å                                                                        a = 5.750(1)Å                                                 b = 3.674(1)Å                                                                         b = 4.5289(8)Å                                                c = 10.130(2)Å                                                                        c = 5.381(1)Å                                                 β = 100.90(1)°                                                                β = 122.61(1)°                        XRD             Phase-pure V.sub.6 O.sub.13                                                               Phase-pure VO.sub.2                               Mean oxidation                                                                               +4.0 (±0.01)                                                                              +4.00 (±0.01)                                state for vanadium                                                            SEM             Needles, sintered                                                                           Bulky powder, non-                                              to form spherical                                                                           sintered                                                        agglomerates                                                  ______________________________________                                    

I claim:
 1. A method for synthesizing an essentially V₂ O₅ -freevanadium oxide having a mean vanadium oxidation state of at least +4 butlower than +5 from NH₄ VO₃ in one step, said method comprising heatingNH₄ VO₃ to effect thermal decomposition thereof while maintaining anautogenous pressure of at least 0.5 MPa.
 2. A method according to claim1 for synthesizing single phase V₆ O₁₃ from NH₄ VO₃, wherein thepressure is kept within the range of 0.5 to 2.5 MPa.
 3. A methodaccording to claim 2 wherein the pressure ranges from 1.0 to 2.0 Mpa. 4.A method according to claim 1 for synthesizing a vanadium oxideconsisting of a mixture of V₆ O₁₃ and VO₂ from NH₄ VO₃, wherein thepressure is kept within the range of 2.5 to 3.5 MPa during thedecomposition.
 5. A method according to claim 1 for synthesizing singlephase VO₂ from NH₄ V₃, wherein the pressure is greater than 3.5 MPaduring the decomposition.
 6. A method according to claim 5 wherein thepressure ranges from 3.5 to 7.0 MPa.
 7. A method according to claim 1,wherein the NH₄ VO₃ is heated to a temperature in the range of 250 to800° C.
 8. A method according to claim 7, wherein the temperature rangesfrom 300 to 800° C.
 9. A method according to claim 8, wherein thetemperature ranges from 425 to 550° C.
 10. A method according to claim1, wherein the NH₄ VO₃ is heated from ambient temperature to reactiontemperature at a rate in the range of 0.1 to 2 K/min.
 11. A methodaccording to claim 10, wherein the rate ranges from 0.5 to 1 K/min. 12.A method according to claim 1, wherein the NH₄ VO₃ is kept at thereaction temperature for a period of from 10 seconds to 24 hours forsynthesis of single phase V₆ O₁₃.
 13. A method according to claim 1,wherein the NH₄ VO₃ is kept at the reaction temperature for a period offrom 2 hours to 5 days for synthesis of single-phase VO₂.
 14. A methodaccording to claim 13, wherein the NH₄ VO₃ is heated in a closedreaction chamber equipped with means for controlling the pressure in thechamber.
 15. A method according to claim 14, wherein said means forcontrolling the pressure in the closed reaction chamber is an adjustablerelief valve.
 16. A method according to claim 1, wherein heating isperformed so that the solid decomposition product and the produceddecomposition gas from the NH₄ VO₃ starting material are permitted toreact.
 17. A method according to claim 16, wherein the volume of NH₄ VO₃powder in said reaction chamber ranges from 1/2 to 9/10 of the reactionchamber volume.